1. Field of the Invention
The present invention relates to the art of cryptography and, more specifically, to the implementation of improved techniques for achieving voice and/or data communication security over a radio and/or telephone channel.
2. Description of the Relevant Art
Cryptography generally relates to the art of protecting sensitive communications against unauthorized access. The proliferation of electronic eavesdropping into sensitive police and military communications has spurred industry to create encryption/decryption devices which prevent such eavesdropping.
Techniques of encrypting or scrambling of radio or telephone signals can take on many forms. Scrambling can go from simple analog encryption to sophisticated digital encryption. Analog scrambling, which makes use of filtering schemes, inverting schemes, time domain schemes and split-band audio schemes is generally easier to incorporate into the radio communication channel than digital scrambling, however, it is much easier for unauthorized users to unscramble an analog-scrambled voice than it is for them to unscramble digital-scrambled voice. Digital encryption, is more difficult to unscramble since it converts the voice to binary bits, scrambles the bits, then transmits the scrambled bits over the communication channel. Thus, the digital system makes unauthorized descrambling or deciphering difficult since all the intruding user "sees" is scrambled bit representation of voice and not scrambled voice itself.
Generally, the three functional blocks used in a digital speech encryption device are: (1) voice coder/decoder (vocoder); (2) encryption/decryption algorithm; and (3) modulation/demodulation methodology. The vocoder functions by first coding analog voice samples and then compressing those samples into a smaller number of binary bits. Thus, when combined with a suitable modulation methodology, the vocoder allows transmission over a communication channel of a smaller, specified bandwidth, and provides intelligible reproduction or decoding via the decode block on the receive end.
First generation vocoders were linear predictive coders (LPC-10) as discussed in Federal Information Processing Standards Publication (FIPS PUB) No. 137, U.S. Dept. of Commerce, NTIS, 5285 Post Royal Road, Springfield, Va. 22161, published on Nov. 28, 1984. The relatively poor voice quality reproduced by LPC coders spurred the industry to develop other vocoders, such as: (1) continuously variable slope delta (CVSD) modulation, described in U.S. Pat. No. 4,167,700, (2) sub-band coding described in T. P. Barnwell et al., "A Real-Time Speech Sub-Band Coder Using the TMS 32010," IEEE Southcon, (1984), and (3) hybrid sub-band coders described in U.S. Pat. No. 4,817,146.
Most of the recently developed vocoders known in the industry are described in "An Evaluation of 4800 BPS Voice Coders" Proceedings of ICASSP (1989). Such vocoders include Vector Adaptive Predictive Coder (VAPC) and Code or Vector Sum Excited Linear Predictive (CELP or VSELP) type coders. Other recent vocoders include Improved Multi-Band Excitation (IMBE) voice coders as described in Proceedings of ICASSP, (1990) and in U.S. Pat. No. 5,081,681 (herein incorporated by reference), and Sinusoidal Transform Coder (STC), as described in U.S. Pat. Nos. 4,885,790 and 4,937,873 (herein incorporated by reference). In addition, there are some very low bit rate (BPS) laboratory voice coders being developed. An exemplary said voice coder is described in Y. J. Liu, "A High Quality Speech Coder at 600 BPS," Proceedings of ICASSP (1990), pp 645-648.
Although vocoders of the conventional art can be adequately interfaced with many different and suitable modulation methodologies depending upon the channel characteristics, typical vocoders must transmit large numbers of bits per second (BPS), and/or utilize sophisticated forward error correction/detection techniques in order to achieve commercial quality voice reproduction. As defined herein "commercial quality voice reproduction" is reproduction which is considered equal to or near equal to the original analog voice normally reproduced through the communication channel. For example, CVSD requires 12,000 BPS while sub-band coding and hybrid sub-band coding requires approximately 9,600 BPS to reproduce commercial quality voice through radios. CELP type vocoders require 4,800 BPS and the IMBE and STC vocoders generally require 2,400 BPS to achieve commercial quality voice reproduction through radios.
Conventional vocoders typically reproduce commercial quality voice in the 0 to 1% bit error rate (BER) condition. With forward error correction, conventional vocoders typically reproduce intelligible voice up to the 5% to 8% BER condition. Thus, the vocoder BPS rates described above do not include the forward error correction/detection required to correct for a constantly varying BER of 0% to 10% encountered in land mobile radio channels (i.e., channels subject to fading and multipath conditions). Typical forward error correction methods such as cycle redundancy checks, parity checks, matrix extraction, bit averaging, Hamming coding, Golay coding, and soft decision, generally require additional or redundant bits be transmitted to ensure data integrity at the receive end. Other methods to help intelligibly receive data include bit interleaving, block or convolution coding.
Forward error correction techniques often require an additional 25-100% BPS be transmitted depending upon the vocoder, with the recent vocoders (i.e., VAPC, CELP, VSELP, IMBE and STC) requiring higher redundancy because of the increased importance of each bit transmitted. While a 10% to 25% redundancy is generally appropriate for a sub-band coder method, a 30% to 100% redundancy is often required for many of the recently developed vocoders described above. Thus, for example, the actual vocoder data rate required for a 2,400 BPS vocoder may be approximately 3,600 to 4,800 BPS in order to operate intelligibly in a land mobile radio. When system overhead is included, such as system synchronization, commands, and encryption initialization, the actual transmitted data rate may be over twice the actual vocoder rate.
The next functional block, or encryption/decryption block, is generally designed either to protect classified information or to protect sensitive but unclassified information. One conventional encryption technique used to prevent eavesdropping of sensitive but unclassified information is the Data Encryption Standard (DES). DES has become the standard algorithm used for sensitive, non-military applications such as, e.g., police communication. DES is fully explained in FIPS Pub No. 46, U.S. Dept. of Commerce, NTIS, 5285 Post Royal road, Springfield, Va. 22161, published on Jan. 22, 1988.
Typically, digital signals are processed in non-return-to-zero (NRZ) format. NRZ, as well as other formats, are often not reliable over communication channels. Thus, the third functional block, comprising the modulation/demodulation methodology, is needed. Modulation/demodulation methodologies are known in the art and can be described in M. Schwartz, Information, Transmission, Modulation and Noise, McGraw Hill, (3rd Edition, 1980); Ferrel G. Stremler, Introduction to Communication Systems, Addison-Wesley Publishing Company, Inc., (2nd Edition, 1982); I. Korn, Digital Communications, Van Nostrand Reinhold Company, Inc., (1st Edition, 1985); and J. Proakis, DigitaI Communications, McGraw Hill, (2nd Edition, 1989). It is important when modulating or demodulating in a communication path, that efficient use be made of the frequency spectrum. As generally utilized for conventional and trunked radios, constant envelope modulation techniques, such as frequency shift keying (FSK), or phase shift keying (PSK), may often cause the communication medium to occupy too wide a transmitted bandwidth for the governmental standard of 15 KHz to 25 KHz spaced radio channels. Also, the transmitted frequency components representing the digitally encrypted signals generally exceed the voice passband of 300-3200 Hz required in conventional radio and telephone systems when operating at the required data rates/hertz for digital secure voice communications.
To aid in the discussion and understanding of the present art, the following terms are defined herein: "conventional radios" are defined as frequency modulated (FM) and phase modulated (PM) analog radios that may contain nonlinear amplifiers and designed for 15 to 25 KHz spaced radio channels as defined in FCC Rules and Regulations Part 90, dated Feb. 15, 1989. Conventional radios may include trunked radios used in a trunked group as defined in FCC Rules and Regulations Part 90, dated Feb. 15, 1989. "15 KHz to 25 KHz spaced radio channels" refer to the separation between the center carrier frequency, measured in Kilohertz (KHz), of each radio channel as designated and listed in FCC Rules and Regulations Part 90, dated Feb. 15, 1989. "Linear Amplifier" is defined as a radio final amplifier, wherein the output is linearly proportional to the input. Generally, a signal requiring a linear amplifier will not work properly with a linearized or nonlinear amplifier. A linear amplifier is generally regarded as an FCC class A amplifier. "Linearized Amplifier" is defined as a radio final amplifier, wherein the output is mostly linearly proportional to the input. Generally, a signal requiring a linearized amplifier will also work properly with a linear amplifier, but not with a nonlinear amplifier. A linearized amplifier is generally regarded as an FCC class AB amplifier. "Nonlinear Amplifier" is defined as a radio final amplifier, wherein the output is not linearly proportional to the input. Generally, a signal that will work properly with a nonlinear amplifier will also work properly with a linear or linearized amplifier. A nonlinear amplifier is generally regarded as an FCC class C amplifier. "Conventional Repeater" is defined as a conventional radio that is used to expand the coverage by receiving and re-transmitting the signal to a satellite receiver, or receivers.
The industry has increased non-linear modulation efficiency using continuous phase modulation techniques, such as continuous phase frequency shift keying (CPFSK) including minimum shift keying (MSK) that have produced bandwidth efficiencies of up to 2 bits/Hz. However, one of the most popular and bandwidth efficient of these methods, MSK, requires a minimum separation of the transmitted frequencies that equals one-half of the BPS rate. Thus, for conventional digital encryption techniques employing MSK and requiring 9,600 BPS, as described in U.S. Pat. No. 4,817,146, a frequency separation of 4,800 Hz is needed thereby requiring frequency components of 2,400 Hz and 7,200 Hz be transmitted. It is believed that this constant frequency separation or shift of 4,800 Hz may exceed the design of some conventional phase modulated radios. An additional problem associated with conventional digital voice encryption modulation techniques is that they often require linearized response amplifiers and filters. Such is the case in U.S. Pat. No. 4,167,700 which describes an improved repeater which contains a "regeneration" means for retiming and reshaping the signal at remote locations without descrambling the signal. This method requires compatibility with linearized amplifiers and thus is not applicable to conventional radios and repeaters. Any of the above reasons, or others, may force consumers to dispose of their conventional radio equipment, including associated repeaters, and purchase new radio systems (i.e., radio equipment and repeaters) in order to use digital encryption/decryption with their radios.
As stated above and discussed in U.S. Pat. No. 4,852,166, another problem that typically arises in conventional digital voice encryption devices is that the transmitted BPS rates require transmission of frequency components that exceed the voice pass-band of 3200 Hz (approximately 300 to 3,200 Hz) specified by the FCC for conventional and trunked analog radio and telephone systems. One such system is described in U.S. Pat. No. 4,817,146. Patent '146 specifies a fairly high transmitted frequency component of approximately 7200 Hz. To compensate for the transmitted high frequencies, a low pass filter is modified or bypassed in the radios and the level of modulation deviation is often lowered to remain within the approved occupied bandwidth of 15 KHz to 25 KHz spaced channels. Oftentimes, lowering the modulation deviation results in a reduction of transmitter efficiency. To compensate for a loss of transmitter efficiency, radio transmitter power must generally be boosted. As shown in the FCC Rules and Regulations, Title 47 Code of Federal Regulations (CFR) Part 22.508 dated Oct. 1, 1988, a low-pass filter is required before the modulator in a conventional and trunked radio that attenuates audio frequency components above the voice pass-band from 3 KHz to 15 KHz by at least a scale of 40 log.sub.10 (f/3) decibels (db) where "f" is the audio frequency in KHz. Thus, the digital encryption device described in Patent '146 (which transmits a 7200 Hz high frequency component), must modify or bypass this low-pass filter because the transmitted signal strength (power) of the high frequency component would be attenuated by approximately 15 db below the low frequency component (i.e., 2400 Hz). This would result in a conventional radio only transmitting the high frequency component at a power level substantially equal to 1/32nd of the low frequency component transmitted power, rendering the device virtually useless.
Also, as discussed in the FCC Rules and Regulations Part 2.202 dated Oct. 1, 1987 and Part 22.507 dated Oct. 1, 1988, the formula to calculate the necessary bandwidth occupied by a typical FM radio emission is B.sub.n =2M+2DK, wherein:
B.sub.n =necessary bandwidth in hertz. PA1 M=maximum modulation frequency in hertz. PA1 D=peak frequency deviation, i.e. half the difference between the maximum and minimum values of the instantaneous frequency (i.e., instantaneous frequency in hertz is the time rate of change in phase in radians divided by two) PA1 K=an overall numerical factor which varies according to the emission and which depends upon the allowable signal distortion: K=1 for typical FM Radio. PA1 B.sub.n =2(7200 Hz)+2(10000 Hz/2)(1) PA1 B.sub.n =14400 Hz+10000 Hz PA1 B.sub.n =24400 Hz or 24.4 KHz PA1 B.sub.n =2(7200 Hz)+2(5000 Hz/2)(1) PA1 B.sub.n =14400 Hz+5000 Hz PA1 B.sub.n +19400 Hz or 19.4 KHz
The maximum occupied bandwidth for the digital encryption apparatus described in Patent '146 when transmitting a 7200 Hz frequency component is calculated as, M=7200 (highest frequency) and D=maximum 10000/2 Hz, or 5 KHz as described in Part 22.507 above, the maximum occupied bandwidth could be:
By lowering the peak deviation (D) to 5000 Hz, which is believed to be the lowest peak deviation that will provide satisfactory results (+/-2.5 KHz), the necessary occupied bandwidth should be:
Thus, the occupied bandwidth of the device described in Patent '146 should fall within the allowable 20 KHz for digital voice transmissions on 15 KHz to 25 KHz spaced channels if the peak deviation is substantially lowered from the maximum 5 KHz allowed.
The FCC Rules and Regulations concerning occupied bandwidth, frequency deviation, voice pass band and resulting radio designations are fully explained in Title 47, Code of Federal Regulations (CFR) Parts 2, 22, and 90 available at the U.S. government printing office.
New digital radio standards are currently being developed by both the U.S. government (Federal Standard 1024) and the public safety community (e.g., Associated Public Safety Communication Officers, Inc. (APCO) Project 25) to achieve increased spectrum efficiency. The new standards may ultimately narrow the channel spacing in a portion of the spectrum to about 6.25 KHz, versus the current 15 KHz to 25 KHz spacing. Examples of modulation methodologies that are being considered to obtain this efficiency are 4-ary FSK, generalized tamed frequency modulation (GTFM), quadrature differential phase shift keying (QDPSK), pi/4 shift QDPSK, C4FM, and QPSK-compatible (QPSK-C). These modulation techniques have not achieved the 6.25 KHz efficiency when designed into conventional radio transmitters without wholesale redesign of the radio itself. It is believed that there are currently no plans or discussions underway that will allow conventional radios to be forward compatible to the new digital radios in the new digital mode, regardless of the occupied bandwidth required by older conventional radios.
As explained above, digital voice coders typically used in digital voice encryption applications remain intelligible in the BER condition. That is, the voice typically remains intelligible even though the received information contains wrong information (i.e., bit errors) in the range of 0% to 8%. However, it is also commonly known that the critical control information must be interpreted correctly, i.e., interpreted error-free by the receive unit in order for the voice to be decrypted properly. Critical control information typically includes system synchronization, encryption initialization, a method of identifying the encryption code key used and a method of identifying units authorized to receive the information.
Typical digital transmission methods of the prior art use either asynchronous methods or synchronous methods at a fixed speed. The fixed speed means the BER condition for the critical control information is the same as the voice information since both blocks are sent at the same speed. However, only the control information must be interpreted error-free. Asynchronous methods include the synchronization bits within the data bits. Synchronous methods separate the synchronization bits from the data bits. However, when a fixed speed is utilized, the device then becomes limited to only the application for which it was designed. A recent application of the new digital radios suggests combining both voice and data into the same bit stream at a fixed speed, but this application has not been shown to be suitable with conventional radios.
Generally, to ensure correct reception when using synchronous techniques, the critical control information is redundantly transmitted such that each subblock of information (e.g., encryption initialization or re-initialization subblocks) is repeated several times whenever the critical control information is transmitted. Also, the entire critical control information block is repeatedly transmitted as often as possible, often more than twice per second, to ensure the receiving units remain in synchronization throughout the message. Retransmitting the critical control information throughout the message also allows those units that are lost to "enter the conversation late" and decrypt the remainder of the message. This is commonly known as "late entry". Also, retransmitting the critical control information throughout the message allows the message to be more secure by continuously re-initializing and changing the encryption mapping via transmission of new encryption initialization vectors (IV).
Redundant transmission of the critical control information is necessary to compensate for the fading and multipath effects associated with land mobile radio channels. These effects, known as Rayleigh fading to those skilled in the art, cause the transmitted signal to fade by an average of -20 db (decibels) for an average length of 45 milliseconds (ms) per occurrence. Also, as the carrier frequency band increases, the occurrence of fades per second increases which typically results in comparatively poorer system reliability at frequency bands such as 800 MHz to 900 MHz versus 150 MHz to 450 MHz.
A conventional digital encryption device that uses synchronous methods is described in U.S. Pat. No. 4,757,536. Patent '536 utilizes a fixed speed and a very redundant "preamble" data format for transmitting the initial critical control information. This redundant preamble cannot be repeated throughout the message without losing a large portion of the transmitted voice data, or without allowing for a late entry so infrequently as to be deemed virtually useless. Thus, subsequent re-synchronization, encryption re-initialization and other control information that allows for late entry in this method is transmitted as header information only once per data frame and thereby is not as reliable as the initial preamble. Where the long preamble format virtually assures reliable initial synchronization at the beginning of a message, one error during the re-initialization of the encryption subsequently transmitted during the same message causes the receiving unit to lose encryption synchronization, or go into a "coast" condition. As described in Patent '536, the "coast" condition occurs whenever a transmitting or receiving unit has previously received control information properly (i.e., attained a steady state) and has subsequently detected an error during re-initialization of the encryption process. The "coast" condition allows the receiving unit to then predict the correct IV and continue decrypting the message.
As taught in our co-pending parent application Ser. No. 07/621,476, a dynamic speed change can be designed to occur between the master prologue (control information) and a subsequent voice or slave prologue (voice block or secondary command/data block) depending upon the command word in the master prologue. The method of application Ser. No. 07/621,476 allows for the control information to be sent at a slower BPS to reduce the BER condition during transmission of the critical control information and allows the BPS to change as required for transmission of the appropriate voice or data information. When comparing the lower versus higher BPS rates for the same BER condition, informal tests have shown the higher BPS rates require a received signal strength of 3 db to 6 db greater than the lower BPS rates, depending upon the actual BPS rates being tested. As such, lower BPS rates provide more accurate transmission reliability. Informal tests have shown a 3 to 6 db improvement in the signal to noise ratio for the same BER condition depending upon the actual BPS rates being tested. As is readily appreciated, data applications, such as information transmitted by data terminal equipment (DTE) must also be interpreted error-free in order to use the information. Most data applications through conventional radios transmit the data in packet format, wherein the same packet of information is repeated until the receive unit responds that the packet was received correctly. The command word as taught in application Ser. No. 07/621,476 instructs the transmitter and receiver to remain at a slow speed for packet transfers of data information.
Approximately 14 million U.S. consumers have conventional radios and repeater equipment approved for the standard 15 KHz to 25 KHz spaced channels, most of which are not presently capable of being converted to include digital encryption capability. While many conventional radios have been produced for foreign countries using 12.5 KHz spaced channels, such conventional radios lack retrofittability with digital encryption techniques described herein. If the foreign format radios could be retrofitted with digital encryption, then greater bandwidth efficiency in both secure and non-secure modes could be achieved thereby providing a transition to the new narrow band digital radio standards using existing equipment. If the end user wishes to transmit data, as well as voice, conventional digital encryption devices cannot automatically transmit and receive secure signals at different data and voice transmission rates and/or data frame lengths. Nor can conventional devices permit the user to alternatively send a secure message through a DTE such as a keyboard, etc., or send a secure voice message at a slower speed for playback at the receiver at full speed.
Thus, a substantial need exists for an encryption/decryption device which can be retrofitted into existing off-the-shelf conventional radios. It also would be advantageous for the device to include re-synchronization capability with the same reliability as the initial synchronization and be capable of inputting into the conventional and trunked radio both control and encrypted voice data within the voice passband of approximately 300-3,200 Hz. Furthermore, if the device can send and receive secure information at varying BPS, the device would not be limited to only one application (i.e., voice or data). It would also be advantageous to provide a control and correction technique that can be designed into or retrofitted into conventional repeaters that controls the repeater functions during digitally encrypted transmissions and error-corrects the critical control information without descrambling the encrypted information. A need also exists for a method and device that can substantially narrow the necessary occupied bandwith below the 15 KHz to 25 KHz spaced channels while being able to use a nonlinear amplifier. Also, a need exists to provide a device which provides forward compatibility of conventional and trunked radios to the new federal standard 1024 and ApCO project 25 digital radio standards in the digital mode.